1. Field of the Invention
The present invention relates to the fixation, or fastening, of artificial joint components.
Presently, orthopedic surgeons most commonly use polymethyl methacrylate (PMMA) cement for fixing artificial joint components to bone. This technique has the advantage that high fixation strength is attained immediately post operatively. The patient can undertake physical activity involving the newly implanted joint within a few days post operatively. This is beneficial for the patient's physical well being because it stimulates circulation and respiration.
However, joint implantation using the PMMA cement has not been entirely satisfactory in the long term. Artificial joints, and their fixation, must withstand large mechanical forces. Especially so in the weight bearing joints: hip, knee and ankle. Transfer of these large mechanical forces from the prosthetic joint to the bone is through a complex structural system when cement is employed. The tensile and compression strengths and the moduli of elasticity vary greatly among the elements in this system: bone, cement and prosthesis. When the prosthesis is metal, the cement is the least strong and the least stiff of the three. The cement is also subject to brittle failure. Failure of prosthetic joint implants is often traceable to failure in the cement fixation. Many hip femoral prosthesis stems have fractured after the supporting cement interface has failed in one way or another. The cement is particularly vulnerable to failure in the proximal femur because this is usually the region of maximum force transfer from the prosthesis to the bone.
Because of the demonstrated long term inadequacy of prosthetic fixation using cement, it has been a continuing objective to achieve direct fixation between the prosthetic structural component and bone. Numerous attempts to achieve this goal have been made over many years. Investigators have employed or proposed:
metal components press fit impacted into prepared bone canals or cavities. These components were tapered pins with both smooth or irregular surfaces, acetabular cups with "petals" or teeth for cutting into the bone, or stems with sintered porous metal surfaces. See for instance U.S. Pat. No. 3,996,625 entitled Artificial Hip Joint with Novel Stem issued to Douglas G. Noiles on Dec. 14, 1976, and U.S. Pat. No. 3,855,638 entitled Surgical Coating issued to Robert M. Pilliar on Dec. 24, 1974. PA1 metal components with porous plastic coatings of several kinds. PA1 metal components with surfaces of a fibrous metal composite, which coating is made from a compressed and sintered metal fiber mesh. PA1 metal components coated with biologically active and accepted glassy material, sometimes called a "bioglass" coating. PA1 porous plastic elements have been tried but their mechanical strength is very low. PA1 ceramic components with and without porous or threaded surfaces, and with biologically active ionic surface treatments. PA1 metal components with threaded stems, and threaded ceramic acetabular cups.
Use of most of the above structures and methods does not permit the initial implantation to achieve intimate mechanical load transmitting relationships between the prosthesis and bone. That is, they are intended to permit bone ingrowth into the porosities or irregularities of the surface of the prosthesis. This bone ingrowth phenomenon is reported to take place in about one to five months in order to achieve adequate structural strength for patient physical activity involving the affected joint. During this time the prosthesis-to-bone interface must be maintained without motion, because it is known that motion at this interface will cause the body to develop soft non-bony tissue at this interface which provides inadequate support for the prosthesis. Therefore, most of the above proposed techniques anticipate restricted patient activity for extended periods. Such restricted activity is not desired for reasons of the patient's overall physical health.
The above mentioned threaded prosthetic stem concept can provide initial intimate load bearing prosthesis-to-bone interface. However, the threaded stem has surface discontinuities which reduce the fatigue endurance strength of the prosthetic component. There are additional difficulties in screwing into the prepared bony canal or cavity the entire prosthetic component to achieve the correct depth of insertion and angle of orientation. For instance, a part of the prosthesis may interfere with a part of the bone when attempting to screw the prosthesis into position.
Results of recent experience with prosthetic joint components with porous metal surfaces which foster bone ingrowth have confirmed that bone reshapes and redensifies itself, by a behavior called "remodeling", to suit the path of load transmission from the prosthesis to the bone. This same experience also demonstrates that it is desirable to transfer a maximum fraction of the total load as close as possible to the normal joint surface in order to encourage the retention of a maximum amount of normal bone mass. For example, a femoral stem prosthesis for a hip joint which provides for bone ingrowth at the distal end of the stem may promote load transfer at that part of the prosthesis with the result that the bone adjacent to the proximal part of the stem will not carry a physiological share of the total load and therefore will become less dense and less strong. While a prosthesis so fixed may function satisfactorily, such a biological change is undesired in the event that the femoral stem prosthesis ever has to be replaced, for any of a number of reasons, in which case the surgeon is forced to deal with an abnormally reduced amount of bone stock in the proximal femur.
There are three principles which are generally accepted to apply to the successful fixation of joint prostheses by direct bone contact and support of the prosthesis. One, the prosthesis must be in contact with sound bone. That is, the bone to which force is transmitted by the prosthesis must have adequate strength to support the applied stresses. This implies that the stress applied to the bone will be within the physiological stress carrying capability of the bone. Two, the prosthesis must be a good fit in the prepared bony cavity. And three, there must not be motion between the prosthesis and the bone.
It is clear that the above three requirements are closely interrelated and very much dependent on favorable geometric relationship between the prosthesis and the bone. It must be true that if a patient's joint and bone structure functioned to any reasonable extent prior to implantation of a prosthesis, then the patient's bone quality is somewhere adequate to support the loads due to that degree of function of that particular joint. The problem then becomes one of providing a prosthesis of the correct shape and size to contact the patient's bone at the optimum interface surface for satisfactory transfer of force from the prosthesis to the bone. Further, the prosthesis must satisfy the above and also fill the space created in the bone with the utmost of congruency in order to inhibit motion between the prosthesis and the bone. It has been reported that bone may grow to fill spaces adjacent the implant of up to 2 mm. Certainly, spaces however small between the implants and the bone do not favor the necessary absence of motion therebetween. Because humans vary so remarkably in physical size and shape, we begin to see that each prosthesis should be custom sized and shaped to suit the bone into which it is to be implanted. Aside from the economic cost of providing a custom prosthesis for each joint of each patient, there is an overriding practical impediment to so doing. The exact dimension for an optimum size and shape of prosthesis cannot be determined before the time of surgery when the bone is opened and its true nature is learned.
The truth of the above may be substantiated by the relative success to date of implantation of joint prostheses using polymethyl methacrylate cement. The cement serves the function of providing a custom prosthesis for the individual bone at the time of implantation. The bone is opened, explored, reamed and broached to create a cavity which is surrounded by bone judged by the surgeon to be of adequate strength to support the forces to be received by the bone. The basic prosthesis, usually metal, is available in an assortment of shapes and sizes, perhaps as many as two dozen. The ulilization of PMM cement to fill the spaces between the prosthesis and the bone is, in fact, the creation of a custom prosthesis for that particular implantation. The mechanical properties of the cement are inadequate to provide a satisfactorily high percentage of successful implants for long term use, however.
Accordingly, a primary object of the present invention is to provide means for fastening a load bearing component of an artificial joint prosthesis to the host's bone in a manner which accommodates a pattern of load transfer from the prosthesis to the bone where a maximum fraction of the total load is transferred to that part of the bone nearest the normal joint surface, by which the prosthetic component provides increased interface load transmitting area of contact with the bone at this subject area.
An additional primary object is to provide a system of components for artificial joint prostheses where a number of variations of sizes and shapes for each component can be combined to create a much larger number of combinations in size and shape of prosthesis assemblies and thereby permit the selection of optimum fit between the prosthesis and the bone at the time of surgery.
An additional primary object is to provide means for fastening intimately the load bearing component of an artificial joint prosthesis with high initial fastening strength without the use of PMMA cement in that part of the bone nearest the joint motion surface, while also permitting the critical structural load bearing component to have generous physical dimensions and contours which either add to, or do not diminish, the fatigue endurance strength of the prosthesis.
A further object is to provide a system which facilitates achieving the correct geometric orientation of the load bearing components to the bone which is independent of the firmness of seating of the fastening means.
A further object is to provide the above advantages while using materials of high biological compatibility, structural strength and elastic compliance.
A further object is to provide a system for prosthesis fixation which is compatible with present day orthopedic surgical practice.
A further object is to provide instruments for use in achieving prosthesis fixation to bone according to this invention.